Pixel based dead time correction

Abstract

A positron emission tomography (PET) apparatus and method employs a plurality of radiation detectors (20) disposed around an imaging region (16) and configured to detect 511 keV radiation events emanating from the imaging region. A calibration phantom is disposed in the imaging region. One or more processors are configured to: acquire and store listmode data of the phantom; measure a random rate for each line of response (LOR) from the listmode data using a coincident 511 keV events detector (34) with a time offset (54); determine a singles rate for each detector pixel from the random event rate, for example via a histogram plotting singles rate for each detector pixel; compute a live time factor of each LOR; compute a dead time correction factor as the reciprocal of the live time factor; and correct images according to the dead time correction factor.

The invention claimed is: 
     
       1. A positron emission tomography (PET) system comprising:
 a plurality of radiation detectors configured to detect coincident radiation event pairs defining lines of response (LORs) emanating from an imaging region and detected by detector pixels of the radiation detectors; and 
 at least one processor configured to:
 cause the radiation detectors to acquire listmode data comprising singles events detected by the detector pixels; and 
 compute a dead time correction factor for each LOR defined by a pair of detector pixels wherein the dead time correction factor for each LOR is computed by determining a random rate for each LOR from the listmode data and determining a singles rate for each detector pixel from the determined random rates and computing a live time LT ij  factor for the LOR defined by detector pixels i and j based on the singles rates S i  and S j  for the detector pixels i and j respectively. 
 
 
     
     
       2. The system according to  claim 1 , wherein the operation of determining a singles rate for each detector pixel comprises solving a system of equations R ij ∝S i *S j , where R ij  is the determined random rate of the LOR defined by detector pixels i and j; the symbol “∝” denotes a proportional relationship; and S i  and S j  are unknown singles rates for detector pixels i and j respectively. 
     
     
       3. The system according to  claim 2 , wherein solving the system of equations R ij ∝S i *S j  includes the at least one processor further configured to:
 generate a histogram map of the singles rate per pixel, wherein the histogram includes a scaling factor. 
 
     
     
       4. The system according to  claim 1 , wherein the computing the dead time correction factor includes the at least one processor further configured to:
 compute the dead time correction factor from the live time factor using 
 
       
         
           
             
               
                 DT 
                 ij 
               
               -= 
               
                 1 
                 
                   LT 
                   ij 
                 
               
             
           
         
       
       where DT ij  is the dead time correction factor for each LOR from i to j. 
     
     
       5. A method for computing dead time correction factor per detector pixel in a positron emission tomography (PET) scanner, the method comprising:
 using PET radiation detectors, detecting a plurality of 511 keV radiation events emanating from an imaging region; and 
 using an electronic data processing device, computing a dead time correction factor for each line of response (LOR) defined by a pair of detector pixels of the PET radiation detectors wherein computing the dead time correction factor includes: 
 determining a measured random rate for each LOR from the detected plurality of 511 keV radiation events using a delay technique which measures coincidences with an added time delay offset; and 
 determining a singles rate for each detector pixel of the PET radiation detectors from the determined random rates by solving a system of equations R ij =2τS i *S j  comprising one equation of the system of equations for each detector pair i and j for which a LOR is defined, where R ij  is the determined random rate of the LOR defined by detector pixels i and j and τ is a coincidence window width and S i  and S j  are unknown singles rates for detector pixels i and j respectively. 
 
     
     
       6. The method according to  claim 5 , wherein solving the system of equations R ij =2τS i *S j  includes the at least one processor further configured to:
 generating a histogram map of the singles rate per pixel, wherein the histogram includes a scaling factor. 
 
     
     
       7. The method according to  claim 5 , wherein computing the dead time correction factor includes:
 compute a live time factor LT ij  for the LOR defined by detector pixels i and j based on the singles rates S i  and S j  for the detector pixels i and j respectively according to LT ij =ƒ(S i )*ƒ(S j ) where ƒ(S i ) and ƒ(S j ) are live time factors corresponding to singles rates S i  and S j  respectively. 
 
     
     
       8. The method according to  7 , wherein computing the dead time correction factor includes:
 compute the dead time correction factor from the live time factor LT ij  using 
 
       
         
           
             
               
                 DT 
                 ij 
               
               = 
               
                 1 
                 
                   LT 
                   ij 
                 
               
             
           
         
       
       where DT ij  is the dead time correction factor for each LOR between i and j. 
     
     
       9. The method according to  claim 5  wherein the detecting comprises acquiring PET imaging data for an imaging subject, and the method further comprises:
 using the electronic data processing device, reconstructing the PET imaging data to generate a PET image of the imaging subject and transforming the PET image to generate Standardized Uptake Value (SUV) data for the imaging subject comprising a parametric SUV image or an SUV value for a region of interest; 
 wherein the reconstructing and transforming includes correcting the PET imaging data for detector dead time using the dead time correction factors for the LORs. 
 
     
     
       10. A non-transitory computer readable medium carrying software for controlling one or more processors to perform the method of  claim 5 . 
     
     
       11. A positron emission tomography (PET) imaging system comprising:
 PET radiation detectors disposed around an imaging region configured to detect radiation events emanating from the imaging region; and 
 a calibration phantom configured to be disposed in the imaging region, the phantom comprising a positron-emitting radioisotope; and 
 one or more processors configured to:
 acquire listmode data of the phantom using the PET radiation detectors as radioactivity of the phantom decays over time; 
 determine a radioactivity level versus singles rate curve based on the acquired listmode data and a known radioactivity decay rate of the phantom over the acquisition of the listmode data; 
 determine from the listmode data a random event rate for each line of response (LOR) connecting two detector pixels of the PET radiation detectors; 
 determine a singles rate for each detector pixel based on the random event rates for the LORs; 
 compute a live time factor of each LOR between detector pixel i and detector pixel j based on the singles rates for the detector pixels i and j; 
 compute a dead time correction factor for each LOR as the reciprocal of the live time factor computed for the LOR; and 
 adjust a coincidence window width of a coincident 511 keV event pair detector of the PET imaging system for each LOR using the dead time correction factor computed for the LOR wherein the coincidence window width is adjusted for each LOR defined by pixel pair i,j according to DT ij Δt, where Δt is the coincidence window width and DT ij  is the dead time correction factor for the LOR. 
 
 
     
     
       12. The system according to  claim 11 , wherein the operation of determining a singles rate for each detector pixel comprises solving a system of equations R ij =2τS i *S j , where R ij  is the determined random rate of the LOR defined by detector pixels i and j; τ is the coincidence window width of the coincident 511 keV events detector of the PET imaging system; and S i  and S j  are unknown singles rates for detector pixels i and j respectively. 
     
     
       13. The system according to  claim 12 , wherein solving the system of equations R ij =2τS i *S j  includes the at least one processor further configured to:
 perform a least squares optimization of the singles rate per detector pixel. 
 
     
     
       14. The system according to  claim 11 , wherein the operation of determining from the listmode data a random event rate for each LOR uses a coincident 511 keV events detector of the PET imaging system with a predetermined time offset large enough to avoid detecting true coincident 511 keV event pairs produced by electron-positron annihilation events.